Method of preparing polymer-fiber composite material and composite material resulting therefrom

ABSTRACT

A method of preparing a composite material in which a dry mixture of a polymeric material and a fiber material is subjected to heating and pressure followed by cooling and reduction of pressure sufficient to melt the polymeric material and to maximize fiber material-polymeric material interactions, while minimizing fiber material-fiber material interactions. The resulting composite material has excellent strength.

FIELD OF THE INVENTION

[0001] The present invention is generally directed to a method of preparing a composite material formed from a polymeric material and a short fiber material to produce an isotropic composite material having excellent toughness in which the material is readily able to dissipate applied forces. The method concerns forming a polymeric material-fiber mixture which is placed in a mold and heated to a temperature sufficient to melt the polymer without adversely affecting the fiber and through applying a specified pressure to the mixture. The composite is then subjected to controlled cooling and release so as to obtain a composite material with the desired properties.

BACKGROUND OF THE INVENTION

[0002] Fiber reinforced composite materials made from polymers are known in the art. Such materials are used for a variety of applications such as door facings, sidings, decorative products, structural components for buildings and other applications which can make use of excellent mechanical performance including strength, fatigue and fracture resistance and the like.

[0003] It is desirable to uniformly distribute the fiber material within a matrix of the polymer in an effort to take advantage of the structural properties of the fiber. However, it is common for the reinforcing fibers to either be destroyed or to suffer deterioration when the composition is prepared. The destruction or deterioration of the fiber material can result from extrusion or milling of the composite material into the structural forms often made from these materials.

[0004] It would therefore be desirable to provide a method of preparing fiber-reinforced polymeric materials in which the full benefits of the fiber reinforcement are realized and especially where the fibers are neither destroyed nor deteriorated during preparation of the composite material.

[0005] It would be a further advance in the art to provide a fiber-reinforced polymeric material which has excellent physical properties and fully obtains the benefit of the fiber reinforcement.

SUMMARY OF THE INVENTION

[0006] The present invention is generally directed to a method of preparing a composite material made from a polymeric material and a fiber material and to composite materials produced thereby. The process produces a composite material having excellent toughness in which the composite material can readily dissipate forces applied thereto.

[0007] In a particular aspect of the present invention, there is provided a method of preparing a composite material comprising:

[0008] a) mixing a dry short fiber material with a polymeric material in powder form to form a mixture of the fiber and polymeric material;

[0009] b) heating the fiber-polymeric material mixture at a temperature and under a pressure sufficient to melt the polymeric material and initiate a fiber-polymeric material interaction in the substantial absence of fiber-fiber interactions to form a pre-composite material; and

[0010] c) cooling and reducing the pressure of the pre-composite material to form said composite material.

[0011] Composite materials produced in this manner are also encompassed by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The following drawings are illustrative of embodiments of the invention and are not intended to limit the invention as encompassed by the claims forming part of the application.

[0013]FIG. 1 is a photomicrograph of a cross-section of a composite material produced in accordance with the present invention;

[0014]FIG. 2 is a graph measuring tensile strength as a function of temperature of a composite material produced in accordance with the present invention;

[0015]FIG. 3 is a graph showing tensile strength as a function of temperature for a composite material produced in accordance with the present invention;

[0016]FIG. 4 is a graph showing tensile strength as a function of temperature for a composite material produced in accordance with the present invention; and

[0017]FIG. 5 is a graph showing tensile strength as a function of temperature for a composite material produced in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention is generally directed to a process for the production of dimensionally stable composite materials made from a polymeric material and short fibers having a length of no more than about 5 mm. The process of the present invention enables the mixture of the polymeric material and the short fibers to interact in a manner which maximizes fiber-polymeric material interactions and minimizes fiber-fiber interactions. Fiber-fiber interactions, especially those involving cellulose fibers, if excessive, can be detrimental to the overall dimensional stability of the resulting composite. Composite materials produced in this manner also form part of the present invention.

[0019] Cellulose fibers are hydrophilic and bond via hydrogen bonding which is highly susceptible to moisture absorption. Thus, the presence of significant moisture adversely affects the strength of the composite. The present process in effect encapsulates the fibers with the polymeric material matrix thereby reducing moisture absorption, hence establishing stronger and more resistant fiber-polymer bonds, which may further be enhanced through the use of coupling agents. The present process produces a composite material in which there is an efficient transfer of stress from the polymeric material matrix to the fiber reinforcement material. The present process is advantageous because it is energy-efficient and cost effective by requiring relatively low heat and pressure to conduct the process.

[0020] In accordance with the present invention, the composite material generally contains no more than about 20% by weight of moisture, preferably less than 15% by weight of moisture, more preferably less than 10% by weight of moisture, most preferably less than 5% by weight of moisture. Moisture absorption is measured in accordance with ASTM D 5229 and D 5229 M-92.

[0021] The process is generally carried out by mixing a dry short fiber material wherein the fibers have a length of no more than about 5 mm with a dry powder polymeric material to form a fiber-polymeric material mixture. The mixture is then heated at a temperature and under pressure sufficient to melt the polymeric material while initiating fiber-polymeric material interactions in the substantial absence of fiber-fiber interactions to thus form a pre-composite material. The pre-composite material is then cooled while reducing the pressure thereof.

[0022] The short fiber material which may be used in accordance with the present invention includes fibers having a length of from about 0.5 to 5 mm as well as particles typically having a maximum dimension of no more than about 0.5 mm. The fiber material is typically made from or derived from a cellulose material. The fibers can therefore typically be selected from native cellulose materials, wood pulp in treated or untreated form as well as synthetic fibers which preferably mirror the characteristics of cellulose materials. In accordance with the present invention, the length of the fiber material is limited to no more than about 5 mm and may be in the form of particles, as previously indicated, having a maximum dimension of no more than about 0.5 mm.

[0023] The polymeric materials which may be used in accordance with the present invention may vary over a wide range and include any polymeric material which can form a polymeric-fiber material interaction. As used herein, the term “polymeric-fiber material interaction” means that the polymeric material and fiber engage in a chemical and/or physical association that secures the fiber to the polymeric material within the composite material. As previously indicated, the present process maximizes polymeric-fiber material interactions while minimizing fiber-fiber interactions where individual fibers engage in a chemical and/or physical association with each other. Fiber-fiber interactions tend to adversely affect the properties of the composite material such as its dimensional stability and strength characteristics.

[0024] As previously indicated, the types of polymeric materials which may be used in the present process vary over a wide range. Preferred polymeric materials include polypropylene, polyvinylacetate, high density polyethylene, polymethylmethacrylate and mixtures thereof.

[0025] In making the composite material in accordance with the method of the present invention, the amount of the polymeric material is typically at least about 50% by weight of the fiber-polymeric material mixture. The amount of the polymeric material will typically range from 50 to 75% by weight depending on the type of polymeric material and the type of fibers that are employed. The selection of a suitable polymeric material will depend on the type of properties desired for the composite material. Physical characteristics associated with known polymeric materials such as the preferred polymeric materials mentioned above are well known in the art. The selection of a suitable polymeric material therefore will initially depend on the desired properties of the composite material coupled with the available short fibers which are used to reinforce the polymeric material. The present method is initiated by the selection of a suitable dry polymeric material as well as short fibers having a length of no more than about 5 mm. Suitable amounts of each of the dry materials is physically mixed with or without the aid of a blender or a mechanical mixing apparatus. Mixing is continued until the fiber material is uniformly dispersed within the polymeric material. Once a uniform mixture is obtained the mixture is then placed in a suitable vessel such as a mold for further processing.

[0026] The fiber-polymeric material mixture within the vessel such as a mold is then heated to a temperature of from about 360° to 380° F. under pressure of from about 80 to 115 psi. The step of heating under pressure is continued until the polymeric material melts without corresponding melting or dimensional change of the fibers per se. Once the heating under pressure step is completed, the resulting pre-composite material is cooled while the pressure is gradually reduced to obtain the composite material. The cooling step is typically conducted until the pre-composite material reaches ambient temperature and pressure.

[0027] The present process produces a composite material in the form of a short fiber reinforced polymeric composite system which is essentially isotropic in that the resulting properties of the composite material are at least substantially uniform in all directions. It is believed that the isotropic nature of the composite material is the result of nearly exclusive fiber-polymeric material interactions in the substantial absence of fiber-fiber interactions. As a result, substantially all of the fiber material is involved in reinforcing the polymeric matrix without degrading the properties of the composite material which may result from fiber-fiber interactions.

[0028] As shown in FIG. 1, a photomicrograph of a cross-section of a composite material produced in accordance with the present invention shows that the fibers (shown in a dark color) are intimately disbursed within the polymer matrix (polypropylene) to facilitate the desired fiber-polymeric material interaction.

[0029] It may be desirable to add to the fiber-polymeric material mixture one or more coupling agents to facilitate an interaction between the polymeric material and the fiber. Suitable coupling agents will vary depending on the selection of the polymeric material and the fiber. Suitable coupling agents include, but are not limited to, polyethylene maleic anhydride, and a range of silanes and the like. Additional coupling agents are known to those of ordinary skill in the art.

EXAMPLE 1

[0030] 198 grams of polypropylene and 102 grams of bleached softwood fibers were formed into a homogeneous mixture containing 66% by weight of polypropylene and 34% by weight of the fiber material. The mixture was placed in a circular mold and then heated to a temperature of 360° F. under a pressure of 100 psi for a time sufficient to melt the polymeric material (e.g. 15 minutes) without corresponding melting or dimensional change of the fiber material. Once the polymeric material was observed to be entirely melted, the mold was allowed to cool to ambient temperature while gradually reducing the pressure to ambient pressure over a period of about 20 minutes.

[0031] The resulting composite material was subjected to dynamic mechanical analysis by cutting square, 2-mm thick smooth strips of the material. The strips were then subjected to a three-point bending test and then scanned from −60 to 100° C. at 2° per minute under 1 Hz and 0.1% strain.

[0032] The strips were then cut at 90° to each other and the mechanical properties shown in Table 1 were measured. TABLE 1 σ_(u) σ_(u) Density Caliper Water abs. E (tensile) (tensile) E (bending) (bending) Example (g/cc) swell (%) (%) (kpsi) (psi) (kpsi) (psi) 1 0.89 3.22 20.65 34 1,319 278 2,795 2 0.89 2.28 5.31 — — — — 3 0.81 5.5 15.98 33 808 149 1,257 4 0.86 0.27 5.15 27 451 162 1,460

[0033] As shown in Table 1, the composite material produced in accordance with Example 1 has excellent ability to dissipate force, or toughness, which in a viscoelastic material is characterized by the loss modulus (E″), storage modulus (E′), or their ratio, E″/E′, tangent δ. The storage and loss moduli characterize the composite material's response to sinusoidal deformation under dynamic mechanical testing (i.e. measuring mechanical properties as a function of temperature). The storage modulus, E′, is the component in-phase with the applied sinusoidal deformation; it relates to the stiffness of the composite material, and represents a measure of how the composite material stores the energy of deformation, and allows the composite material to regain shape after deformation. That is to say, it represents the elastic behavior of the composite material, or the solid-like properties it displays. The loss modulus, E″, however, is the component out-of-phase with the applied sinusoidal deformation, and relates to the damping ability of the composite material. It indicates how the composite material dissipates the energy used to deform it, and, thus, represents the viscous nature of the composite material, or its liquid-like behavior. The ratio of loss-to-storage moduli is represented by the loss tangent, tangent δ.

[0034] As shown in FIGS. 2 and 3, the stiffness and ability of the composite material to dissipate stress remains fairly constant over the range −60° C. to 100° C. (which is beyond typical functioning end-use performance from −20° C. to 60° C.). The peak of the tangent δ plot in both figures represents the glass transition, or where E′ and E″ change rapidly. Higher storage modulus for the described composite material indicates its superior rigidity. The data shown in FIGS. 2 and 3, as well as the results shown in Table 1, indicate the exceptional dimensional stability and structural integrity of the composite material over the wide temperature range. The composite material of Example 1 exhibits the most superior mechanical performance, as compared to the examples below, owing to the optimal combination of fiber reinforcement polymeric-matrix interaction and uniformity of distribution.

EXAMPLE 2

[0035] The procedure of Example 1 was followed except that 50% by weight of the polypropylene material was combined with 50% by weight of cellulose fibers in the form of particles have a maximum dimension of less than about 0.5 mm. A dynamic mechanical analysis test was conducted on the composition Example 2 in the same manner as in Example 1 and the results are shown in Table 1.

[0036] As shown in FIGS. 3 and 4, the E′ and tan δ values are fairly consistent over the entire temperature range of −60° C. to 100° C. As in the case of Example 1, the response of the cellulose particle-polymeric material composite to dynamic mechanical testing is similar: stiffness and the ability of the composite material to dissipate stress remains fairly constant over the temperature range of −60° C. to 100° C. The composite material thus produced is highly isotropic; however, the reinforcement (less than 0.5 mm) is less capable (as compared to Example 1) in sustaining high structural integrity beyond the composite material's yield point. The composite material is therefore more brittle, but has a very low rate of water absorption due to the high degree of material isotropy.

EXAMPLE 3

[0037] The same process as Example 1 was repeated using 50% by weight of polypropylene and 50% by weight of short fibers of having a length of no more than about 5 mm. The results are shown in Table 1. As indicated above, the composite material thus produced shows adequate mechanical response, superior to the mechanical response of Example 2, but less than the mechanical response of Example 1.

EXAMPLE 4

[0038] The same procedure as followed in Example 1 was repeated for preparing a composite material containing 75% by weight of polypropylene and 25% by weight of small fibers having a maximum length of 5 mm. The mechanical properties of the resulting composite material are shown in Table 1. The use of higher matrix content in this Example as compared to Example 3 roughly indicates a similar mechanical response. However, water absorption is reduced due to better encapsulation of the cellulose fibers by the polypropylene. It should be noted that the composite materials of Examples 3 and 4, while similar, would be more suitable for situations requiring higher durability or dimensional stability/control, respectively. 

What is claimed is:
 1. A method of preparing a composite material comprising: a) mixing a dry short fiber material with a polymeric material in powder form to form a fiber material-polymeric material mixture; b) heating the fiber material-polymeric material mixture at a temperature and under pressure sufficient to melt the polymeric material and initiate a fiber material-polymeric material interaction in the substantial absence of fiber material-fiber material interaction to form a pre-composite material; and c) cooling and reducing the pressure of the pre-composite material to form said composite material.
 2. The method of claim 1 wherein the short fiber material has a length of up to 5 mm.
 3. The method of claim 1 wherein the short fiber material is in the form of particles having a maximum dimension of no more than about 0.5 mm.
 4. The method of claim 1 wherein the fiber material-polymeric material mixture is heated to a temperature of from about 360° to 380° F.
 5. The method of claim 1 wherein the fiber material-polymeric material mixture is subjected to a pressure of from about 80 to 115 psi.
 6. The method of claim 1 wherein step (c) lowers the temperature and pressure of the pre-composite material to ambient temperature and pressure.
 7. The method of claim 1 wherein the amount of the polymeric material is at least about 50% by weight of the fiber material-polymeric material mixture.
 8. The method of claim 1 wherein the amount of the polymeric material is from about 50 to 75% by weight of the fiber material-polymeric material mixture.
 9. The method of claim 1 further comprising adding a fiber material-polymeric material interaction enhancing agent to the fiber material-polymer material mixture.
 10. The method of claim 9 wherein the fiber material-polymeric material interaction enhancing agent enhances the coupling of the fiber material to the polymeric material.
 11. The method of claim 9 wherein the fiber material-polymeric material interaction enhancing agent is selected from the group consisting of polyethylene maleic anhydride, silanes, and combinations thereof.
 12. The method of claim 1 wherein the fiber material is selected from the group consisting of a native cellulose material, treated wood pulp, wood pulp, synthetic fibers and combinations thereof.
 13. The method of claim 1 wherein the polymeric material is selected from the group consisting of polypropylene, polyvinylacetate, high density polyethylene, polymethylmethacrylate and combinations thereof.
 14. A composite material produced by the process of claim
 1. 15. A composite material comprising fibers having a length of no more than about 5 mm in combination with a melted polymeric material in which the composite material contains no more than about 20% by weight of moisture.
 16. The composite material of claim 1 in which the composite material contains no more than 15% by weight of moisture.
 17. The composite material of claim 1 in which the composite material contains no more than 10% by weight of moisture.
 18. The composite material of claim 1 in which the composite material contains no more than 5% by weight of moisture.
 19. The composite material of claim 15 wherein the fibers having a length of from about 0.5 to 5 mm.
 20. The composite material of claim 15 wherein the fibers are made from or derived from a cellulose material.
 21. The composite material of claim 15 wherein the polymeric material is selected form the group consisting of polypropylene polyvinyl acetate, high density polyethylene, polymethyl methacrylate and mixtures thereof.
 22. The composite material of claim 15 wherein the amount of the polymeric material is at least 50% of the composite material. 